1. Field of the Invention
The present invention relates to a magnet wire with improved windability and insertability which, in addition, is bondable at elevated temperatures, including the range 185.degree. C. to 200.degree. C. and includes a topcoat around and along the length of a basecoat provided over a conductor, wherein the topcoat is a polyamide selected from the group consisting of polyundecaneamide, polydodecaneamide and mixtures thereof.
2. Description of the Art
An insulated copper or aluminum wire used in the coils of all types of electromagnetic machines, such as windings of motors, solenoids and transformers, is known as magnet wire. The most widely used types of insulation for magnet wire include enamel, natural and synthetic fibers, glass and paper. Depending upon the type of insulation, magnet wire can be classified at temperature indices from 105.degree. C. to 220.degree. C.
The insulation provided on magnet wire often comprises a dual system which includes a basecoat and a topcoat. The basecoat material is usually chosen for its ability to perform certain functions, such as heat stability, solderability and solvent resistance. Common basecoat materials are polyesters and polyurethanes, although epoxies, polyacrylics, polyimides and amide-imide coatings are also used for basecoats. The term basecoat, as used herein, also includes combinations of the aforementioned materials.
Certain nylons, nylon 6,6 (poly-hexamethyleneadipamide) and nylon 6 (polycaprolactam) in particular, have been employed as a topcoat for magnet wire. Because of the low coefficient of friction of 0.17 (dynamic, film-on-film) for nylon 6 and 6,6 films, such insulated wire exhibits increased windability over other conventional topcoats. The use of magnet wire topcoated with nylon 6 or 6,6, however, may pose several problems to the end user.
First, manufacturers of motors and the like often bond the wound wire in place. The most common method of bonding nylon 6 or 6,6 topcoated magnet wire coils is by dipping the entire coil into a varnish bath and baking the varnish on the coil. Such dipping and baking operations are not only time consuming and expensive, but also result in undesirable solvent emissions and fumes that must not be released into the atmosphere. The ideal topcoat would be a self-bonding type, such as polyvinylbutyral coated magnet wire. By heating polyvinylbutyral above approximately 100.degree. C., the coating softens, flows and bonds the windings in place. Polyvinylbutyral overcoated magnet wire is limited to use in low thermal class systems because of its low softening temperature. Magnet wire applications requiring a thermal class rating of at least 130.degree. C. preclude the use of self-bonding polyvinylbutyral. It would not be practical to heat bond nylon 6 or 6,6. Heating nylon 6,6, for example, to its melting temperature of about 250.degree. C. would exceed the thermal resistance of the basecoat and other components of the system, such as slot liners. Furthermore, nylon 6,6 rapidly degrades in air at this temperature.
A second disadvantage of nylon 6 or 6,6 topcoats or films is the fact that such materials absorb water. Water absorption decreases the electrical performance of the wires, but improves film flexibility. However, nylon 6 and especially nylon 6,6 becomes brittle when they lose moisture resulting in decreased windability and increased insulation cracking problems.
Thirdly, although a nylon 6 or 6,6 topcoat typically improves the windability and insertability of magnet wire over other magnet wire insulating materials, winding and inserting problems are still encountered. The coefficient of friction (dynamic, film-on-film) of nylon 6,6 is 0.17. A topcoat material exhibiting a lower coefficient of friction, even an improvement of 0.01, would significantly increase the windability and insertability of the magnet wire. The prior art, such as U.S. Pat. No. 3,632,440, recognizes this advantage and teaches that use of film forming polysiloxane resin in the topcoat will outperform nylon insulation in that the coefficient of friction of such magnet wire measures 0.14. However, in most electrical systems, the presence of silicone is intolerable.
Accordingly, an improved magnet wire is desired that is self-bonding at a temperature that does not harm the basecoat and other system components, is moisture resistant and exhibits improved windability and insertability. Such improvements should not significantly affect the flexibility, abrasion resistance or heat shock resistance of the magnet wire.